1) Field of the Invention:
The present invention relates to heat-resistant bonding materials suitable for use in die-bonding of pellets for IC (an integrated circuit) and LSI (a large-scale integration).
2) Description of the Related Art:
In a semiconductor device, a semiconductor chip is bonded to an insulating substrate or to a metal frame which serves as a support. Further, the semiconductor pellet is electrically connected to leads formed on the substrate, for example, by way of wires. The resulting assembly is then hermetically sealed with a metal, ceramic cap, molding resin or the like.
As a bonding method for semiconductor pellets, it has been the practice to use an epoxy resin as a bonding layer, to form an Au--Si eutectic by heat treatment with an Au foil and then to use the same as a bonding layer, or to employ glass as a bonding layer. These methods are described in detail in Publication of Unexamined Japanese Patent Application (Kokai) No. 54-69065 and E.C.C. Proceedings, 37, 96-109 (1987).
There is a strong demand for ceramic CMOS packages for large computers in recent years. Their developments are now under way. Large chips of 10 mm.times.10 mm or greater are used in such ceramic packages. These large chips have however developed inter alia the following problems:
(1) Chipping or breakage of pellets due to the concentration of stresses in bonding layers upon their bonding; and
(2) Deterioration of the bonding layers because of the thermal history upon glass sealing.
The problem (1) tends to take place especially when an Au-Si eutectic or glass layer, which forms a rigid bonding layer, is used. The problem (2) tends to occur when an epoxy resin is used as a bonding layer.
As adhesive materials which may be able to overcome these problems, the high heat-resistance polyimide resins disclosed in Publication of Unexamined Japanese Patent Application (Kokai) Nos. 61-264023 and 62-199674 have attracted attention.
To obtain desired bonding strength, these polyimide resins require the application of a pressure of 2.0-3.9 MPa upon bonding. Application of such a large compression bonding force upon die bonding of a pellet however results in deleterious effects, such as chipping or breakage, cracking of passivation film or disconnection a device.
With a view toward avoiding this problem, pellets are bonded by potting using a polyimide resin in the form of a varnish. This method has however found to involve the problem that the bonding material adversely affect the quality of devices by (1) an overflow of the varnish to the wire bonding areas of package bases and (2) an increase of thermal resistance due to the occurrence of voids within the bonding layer as a result of evaporation of the solvent upon heat setting of the varnish.